The WA Electoral Commission has determined the winners of the state's three undecided Lower House seats in State Parliament with all incumbents being returned.

One week after the state election, the winners can finally celebrate.

Labor's Mick Murray saw off a challenge from the Liberals' Jaimee Motion.

Mr Murray thought it was all over on Tuesday, but managed to hold onto his seat of Collie-Preston by 59 votes.

He says it was the toughest fight of his political career.

"Relieved is probably the best way to put it," he said.

Labor won Midland, with Michelle Roberts hanging on to her seat by just 23 votes.

But the Liberals tasted victory too, with Graham Jacobs securing a 125 vote win over the Nationals' Colin de Grussa.

The results give the Liberal and National parties a total of 38 seats to Labor's 21.

WA Electoral Commission staff are continuing to work across the weekend to finalise numbers for Upper House seats.

The WAEC says it is possible counting in the Agricultural region could be finalised by Sunday night with results in other regions expected in the following days.